Touchscreens may apply of a number of varying technologies for obtaining the touch-sensitive functionality. Among various other potential options e.g. capacitive, resistive, infrared, optical imaging (camera-based), acoustic, and hybrid solutions are feasible.
Quite a few of the current infrared solutions implement an unrestricted optical connection between light sources and receivers, whereupon a finger or stylus deforming the screen cover, which overlays the display and light beams of the detection arrangement, then interrupts one or some of the beams for location detection purposes, or alternatively, the finger or stylus may directly interrupt the beam(s) in versions having no transparent overlay plate. Touch detection capability of an infrared solution may also rely on FTIR (frustrated total internal reflection) phenomenon, wherein the amount and distribution of light reaching the detectors is dependent on the disturbance introduced to a lightguide surface by e.g. a fingertip or a stylus applied for control input purposes so that a phenomenon called ‘Frustrated Total Internal Reflection’ (FTIR), i.e. light (energy) leakage, takes place.
FIG. 1a illustrates the FTIR phenomenon in connection with a lightguide. In the scenario of sketch 102, a ray of light 104 is incoupled to the lightguide 106 at one end in such a way that it propagates inside the lightguide by totally reflecting 108 from the walls thereof. The ray 104 finally exits the lightguide 106 via the far-end thereof. In the subsequent scenario of sketch 110 a fingertip 112 is placed on the lightguide top surface so that at least part of light is absorbed (in the finger), diffusively reflected, and/or refracted 114 at the particular location of contact in different directions, e.g. outside the lightguide 106, and possibly only a portion of the original ray is specularly reflected and ultimately reaches the far-end as earlier. Note that the ray 104 is represented thinner in the figure after the interaction point 114 in the original direction of propagation, which is used to depict the aforesaid effect of energy loss (the star and small arrows in the figure) due to the FTIR taking place. FTIR-based light leakage/loss may be then detected and utilized in position sensing applications such as touchscreens that generally exploit the TIR effect of light.
Some theoretical aspects behind the (F)TIR phenomenon are next briefly derived from the well-known Snell's law. Considering the standard representation thereof, i.e. n1 sin θ1=n2 sin θ2, wherein n's represent the indexes of refraction on the both sides of a medium border whereby θ's represent the incident angles relative to the normal of the medium border, respectively, and then setting θ2, which thus refers to the refracted ray, as 90°, we obtain a so-called ‘critical angle’ for the incident angle θ1 via the modified equation of Snell's: n1 sin θ1=n2. Typically, when light enters a boundary region between two media, portion thereof is refracted and portion is reflected. However, for the angles of incidence larger than the critical angle, the light will be substantially completely reflected at the medium border, wherein the angle of incidence is equal to the angle of reflection according to the law of reflection. A general prerequisite for the total internal reflection to occur is the propagation direction of light from the medium with a higher index of refraction (optically denser material) into the medium with a lower (˜less dense material) index, i.e. n1>n2.
For instance, in the example of FIG. 1a placing a finger on the lightguide may change the refractive index of the neighbouring medium radically (assuming the original neighbouring medium 2 is e.g. air with refractive index of about 1 meanwhile the index of a human skin may be about 1.4-1.5, for instance) potentially causing the considerable coupling loss between the lightguide incoupling and outcoupling ends due to the increase in the critical angle and possibly even the requisite for total internal reflection n1>n2 not holding true anymore, whereupon many rays falling between the range of old and new critical angles may now actually refract instead of reflection. In practice, e.g. the surface roughness of the lightguide affects the fact that the sealing between it and the finger, which has surface irregularities as well, is not perfect, i.e. some air still remains in between. However, thanks to a so-called evanescent wave coupling, wherein evanescent waves extending from the lightguide across the lightguide-external medium border (e.g. glass-air interface) to a further nearby (order of light wavelength) medium, such as the human finger, having a higher refractive index compared to the sandwiched medium, such as air, and pass energy thereto as well like in quantum tunneling, a perfect seal is not even required for coupling purposes. Thus, the total internal reflection is said to be ‘frustrated’.
Publication US20060114237 discloses an FTIR touch screen provided with infrared emitters/receivers. The concept of the solution of publication is visualized in FIG. 1b as an isometric sketch. The disclosed arrangement utilizes a strobing-type scan, wherein a plurality of light emitters 120, 122 and receivers 118, 124 have been organized along the edges of the lightguide 106 and they are sequentially activated/deactivated in emitter-receiver pairs (˜note the broken lines illustrating the main direction of light propagation between an emitter and receiver of a single pair) so that a touch at a certain location on the lightguide can be detected by the reduced amount of light at the receiver of an active emitter-receiver pair due to the FTIR phenomenon.
Without any intention to deny the advantages and benefits offered by currently available touchscreen or corresponding solutions in providing more sophisticated UI means over more conventional options such as keyboards and mouses, certain problems still exist therewith naturally depending on each particular use case. For example, traditional touchscreens are often somewhat pricey to implement and manufacture, and they also take a considerable amount of space in the end product without forgetting the induced additional weight, which must be thus taken into account in the very beginning of the overall R&D project. The touchscreens may even consume suprising amount of extra power e.g. in the context of mobile devices. Further, additional structures such as light incoupling and/or outcoupling structures, e.g. prisms, reflectors, gratings, etc, may be required to funnel the light from the light source to the lightguide and/or from the lightguide to the receiver, respectively. Such structures require some more design work and may add to the end product weight, coupling losses, and price among other factors.